1. Field of the Invention
The present invention relates to a process and to an apparatus for the gasification of coal by supplying coal and reactive gas to the top of the reactor, the gas and solid flows moving downwards cocurrently over most of the reactor, and completing the reaction by the countercurrent introduction of a quantity of reactive gas at the base of the reactor. The reaction takes place in fixed bed, under pressure and with unfused ash.
In the present specification, the term "coal" refers to any carbonated substance with a wide range of particle sizes, without having to resort to an upstream preparation of coal, such as, crushing, grinding, sieving and the like. Moreover, the coal may be of any composition, without having to resort to an equipment downstream for the purification or the recovery of byproducts (sulphur-containing compounds, tars, phenols and liquid hydrocarbons). The process of the invention enables all harmful quantities of by-products to be removed or converted "in situ" during the gasification.
In the present specification, the term "fixed bed" refers to a non-turbulent bed which only undergoes a slow downward movement under the influence of gravity.
2. Description of the Prior Art
Most low-pressure or high-pressure fixed bed coal gasification processes used on an industrial scale are countercurrent systems.
A good representative example is the LURGI process, the gas generator of which is shown in FIG. 1. In the LURGI process, size-graded coal with low caking capacity is introduced at the top of the reactor, from there it moves downwards in fixed bed under the influence of gravity and ash is extracted at the base. Steam and oxygen are introduced at the base of the reactor and they are allowed to rise countercurrently and the gases produced are evacuated at the top of the reactor.
This countercurrent process has the advantage of providing a fairly good heat recovery from the gases resulting from the reaction, by giving up a part of the heat to fresh batches of material charged into the furnace. Additionally, because of the countercurrent circulation of fresh oxidizing gas, the reaction is more or less complete. Therefore, there is a minimum loss of active substance in the ash.
On the other hand, given that the hearth of the reaction is at the bottom of the reactor and that the temperature decreases with height towards a point where there is fresh coal, there is a region where the temperature is in the vicinity of 500.degree. C. and where a slow increase in the temperature promotes the formation of a liquid phase which causes the caking of coal particles preventing a regular downward movement of ash. In order to avoid this caking, the LURGI process requires the choice of coal with low caking capacity upstream. Additionally, within the gas generator itself, it was necessary in the LURGI process, to provide a mixing arm to stir up the fixed bed of coal.
If the size of coal is not uniform, an irregular distribution of gases takes place, because of which there is a need for the choice of a size-graded coal (for example, having a particle size between 10 and 30 mm) and for an outfit for sieving and evacuating the fine particles upstream.
The hearth of the reaction is situated at the bottom of the reactor. In order to prevent the temperature there from rising too high, the grid for distributing the oxidizing gas must be cooled by using a large excess of steam, which results in a negative effect on the energy balance and profitability.
The decreasing temperature gradient in the reactor causes by-products to be evolved gradually and to be mixed with the gas produced and 70 to 95% of the sulphur contained in the coal may thus be found in the form of sulphur-containing gaseous compounds in the gas evacuated at the top of the reactor. Therefore, this gas necessitates a purification outfit downstream which must not only remove the sulphur compounds, but also other hindering non-gaseous products such as tars, liquid hydrocarbons, ammonia and the like, from the gas produced.
U.S. Pat. No. 3,920,417 describes a gas generator in which coal and reactor gas descend cocurrently into the lower part of the reactor. In contrast, the upper part of the reactor consists of a region for the countercurrent pre-heating of coal, with a very hot hearth at the base of this region and a temperature which decreases towards the point where fresh coal is present, the fresh coal being introduced at the very top of the reactor. All the disadvantages of the countercurrent process are therefore encountered in this region, i.e. caking and the formation of tars and sulphur-containing gases. Additionally, this process is limited to the use of non-caking coal of a well defined size, and the tars and other undesirable liquids are reinjected into the reactor by means of a steam injector thereby linking the quantities of steam to those of the tar formed.
The hearth of combustion in the gas generator of this patent produces a very high temperature (approximately 1800.degree. C.). In fact, it is desired to produce fused ash there, but, as the steam is introduced downstream of the oxidizing gas and an endothermic reaction follows therefrom, there is every reason to fear that the liquid ash generated upstream becomes cooled, inevitably causing the solidification of the ash, the formation of large-sized clinkers and the blockage of the reactor. Moreover, brick baffles which serve as filters for the liquid ash in the process of solidification will soon be clogged and gas cannot pass uniformly through the reactor towards its point of evacuation. Finally, as the gas generator does not contain a lock, it cannot be operated under pressure.
German Pat. No. 54,995 describes a fixed gas generator in which coal and the major part of the reactor gas are introduced through the open top of the gas generator and descend cocurrently in the latter. In order to avoid caking, additional quantities of air, steam and coal are injected at other lower levels. The gases produced are extracted with a gas suction extractor and therefore, under reduced pressure. This gas generator which is equipped with sliding gates, a water trap and poker holes for removing blocked clinkers cannot operate under pressure. This patent describes the possibility of heating the air with overheated steam, but the risk of caking implies that the temperature of the gas remains below 500.degree. C. Therefore, this gas generator does not provide means which prevent the clinkers from binding together and the coal from caking. On the other hand, the process of this patent, in which a final countercurrent reaction phase is not provided for, clears, with the ash, residual amounts of carbon, which represent a loss or require recycling.
French Pat. No. 783 087 describes a wood gas generator for vehicles with OTTO engines. The combustion therein takes place in a fixed bed and cocurrently. The hearth is divided into several compartments for a better distribution of air injection nozzles and for a loosening of the incandescent bed. In addition to compartmentation of the hearth, the specification of this patent emphasizes the compartmentation of the upper part of the gas generator into several chambers (claim 10), one above the other, at least one of which comprises a drying region and a distillation region. The distillation region produces tars and other undesirable liquid products which must be recycled to ensure their removal, which remains partial, this being a major disadvantage. Additionally, the process of the French Patent can only be carried out at very low pressure (1.5 bar), does not comprise a lock, does not include any continuous ash extraction device and must therefore be interrupted regularly in order to clear the ash and to be cleaned before introducing a fresh charge. Being designed for using wood, if the wood had to be replaced with coal, the caking of the latter in the drying region would be inevitable, as there is no provision for preventing this from occurring.
German patent No. 1 048 658 describes six reactors which operate cocurrently with dry ash and which make use of coal of fine particle size. In each reactor, the coal is fluidized in turn by countercurrent blowing, across the incandescent bed, with a view to bringing about a separation of particles by gravity, so as to direct the ash downward. The blowing is preceded and followed by a blowing of steam above the bed in order to remove oxygen and prevent explosive mixtures from being formed. This blowing is carried out in all the gas generators simultaneously.
U.S. Pat. No. 1,505,065 describes a gasification of soft coal with a view to producing a gas which has a low calorific value, a high hydrogen content and as low a carbon monoxide content as possible. The gas generator comprises two regions: an upper carbonization region and a lower production region. In the carbonization region, coal is loaded at the top and flows downwards. The reactive gas (air+excess steam) is mainly introduced between the two regions, so that it produces an incandescent hearth in the middle of the gas generator and the reactive gas rises in part countercurrently in the carbonization region, gradually heating the fixed bed of coal above and another part of this gas moves down cocurrently with the incandescent coal. Injection of additional amounts of reactive gas is provided for in this lower production region. Given that this process comprises an upper region which is heated gradually, it is not suitable for caking coals. It cannot operate under pressure, as the ash and the dusts are cleared laterally. It cannot prevent ash from being carried away in the gases towards stackings which will clog rapidly; cleaning them is difficult.
European Patent application No. 0,089,329 describes a process for coal gasification under pressure, but it comprises two reactors which operate alternatingly, one at a time. This process has several disadvantages. As it has two reactors and many valves, it is much more expensive to implement and difficult to use. As only one reactor is used at a time, it requires a sweeping with steam after each cyle, that is, every 2 to 4 minutes, in order to separate the reactive gases; that is, air or oxygen and steam and the combustible gases of reaction by purging the plant. Cyclic reversal and constant fluctuations in temperature make the refractories become fragile, making it necessary to replace them more frequently. Finally, the heat contained in the gases produced must necessarily be recovered in this case, which does not allow the hot gases leaving the gas generator to be used directly, as, for example, in the synthesis or in the production of the reducing gas in the steel industry, without using an independent and expensive heating device.